The present invention relates to communications, and more particularly, to radio communications networks and terminals.
In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. User equipment units may include mobile telephones (“cellular” telephones) and/or other processing devices with wireless communication capability, such as, for example, portable, pocket, hand-held, laptop computers, which communicate voice and/or data with the RAN.
The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “NodeB” or enhanced NodeB “eNodeB”, which can be abbreviated “eNB.” A cell area is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with UEs within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN, short for UMTS Terrestrial Radio Access Network, is a collective term for the Node B's and Radio Network Controllers which make up the UMTS radio access network. Thus, UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units.
The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. In this regard, specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within 3GPP. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
FIG. 1 is a simplified block diagram of a Long Term Evolution (LTE) RAN 100. The LTE RAN 100 is a variant of a 3GPP RAN where radio base station nodes (eNodeBs) are connected directly to a core network 130 rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are performed by the radio base stations nodes. Each of the radio base station nodes (eNodeBs) 122-1, 122-2, . . . 122-M communicate with UEs (e.g., UE 110-1, 110-2, 110-3, . . . 110-L) that are within their respective communication service cells. The radio base station nodes (eNodeBs) can communicate with one another through an X2 interface and with the core network 130 through S1 interfaces, as is well known to one who is skilled in the art.
The LTE standard is based on multi-carrier based radio access schemes such as Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The OFDM technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which avoids having demodulators see frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
FIG. 2 illustrates a resource grid for frequency and time resource elements (REs), where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions may be organized into radio frames of 10 ms, and each radio frame may consist of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 3.
One or more resource schedulers in the LTE RAN 100 allocate resources for uplink and downlink in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The LTE Rel-8 standard has recently been standardized, supporting bandwidths up to 20 MHz. 3GPP has initiated work on LTE Rel-10 in order to support bandwidths larger than 20 MHz and support other requirements defined by IMT-Advanced Requirements. Another requirement for LTE Rel-10 is to provide backward compatibility with LTE Rel-8, including spectrum compatibility. This requirement may cause an LTE Rel-10 carrier to appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it can be particularly important to ensure efficient use of the wide carrier by legacy terminals, such as by enabling legacy terminals to be scheduled in all parts of the wideband LTE Rel-10 carrier. One way to obtain this may be by means of Carrier Aggregation. Carrier Aggregation refers to an LTE Rel-10 terminal being configured to receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. The same structure as Rel-8 implies that all Rel-8 signals, e.g. (primary and secondary) synchronization signals, reference signals, system information are transmitted on each carrier. FIG. 4 graphically illustrates an exemplary 100 MHz Carrier Aggregation of five 20 MHz CCs.
Referring to FIG. 4, the number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the numbers of CCs in downlink and uplink are different. It is important to note that the number of CCs offered by the network may be different from the number of CCs seen by a terminal. For example, a terminal may support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access a LTE Rel-10 terminal may operate similarly to a LTE Rel-8 terminal. Upon successful connection to the network, a terminal may—depending on its own capabilities and the network—be configured with additional CCs in the UL and DL. Configuration is based on Radio Resource Control (RRC). Due to the heavy signaling and rather slow speed of RRC signaling, a terminal may be configured with multiple CCs even though not all of them are currently used. If a terminal is configured for multiple CCs it may have to monitor Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH) for all DL CCs. However, such terminal configuration may necessitate use of a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption.
With multiple CCs used to provide downlinks from a RAN to a wireless terminal over respective different frequencies, the wireless terminal may be required to synchronize and/or configure each of the multiple CCs received over each of the different frequencies.